Closures of the so-called "floating disk" type have recently been developed for use in the vacuum packing of food products. Such closures have a shell which threads onto a container and which holds an axially movable or "floating" disk that forms the seal with the container finish. The container is partially evacuated above the food product, and pressure differential force acts downwardly on the disk to hold it in sealing engagement with the container. Normally it is this differential pressure force which maintains the seal, more than any force applied by the closure shell to the disk. Depending upon the degree of vacuum and the area of the container mouth, this downward force can be so large as to make it undesirably difficult to lift the disk to break the vacuum.
In order to facilitate lifting the disk to break the vacuum, it is known to provide lifting means on the inside of the closure shell which, as the closure is twisted to open the container, is moved upward toward the disk and comes into engagement with the edge of the disk to lift it to break the vacuum. In some closures the upper end or ends of the internal thread or threads on the shell engage and lift the disk. However, because of the angulation of the threading the upper thread end engages the disk only over a very small area. Therefore considerably greater pressure must be applied through the small thread end area in order to overcome the downward force on the disk. This can require excessive opening torque and can result in deformation of the disk. Especially in the case of a single start or a two start thread, this area is not always sufficient to prevent the thread end from stripping past the disk (which is held down by vacuum), leaving the disk behind on the container finish instead of being lifted by the thread end.
To avoid that problem, it is also known to utilize lifting means on the inside of the skirt above the upper end of the shell threads. These means are a continuous or interrupted bead which engages a large portion or all of the periphery of the disk, rather than just localized points, so that the disk can be more positively lifted to break the vacuum. The lifting bead in such a closure is positioned above the external threads of the container so that the container threads do not interfere with the upward movement of the bead as the closure is twisted on the container. Such a floating disk closure is shown in Szczesniak U.S. Pat. No. 4,770,306.
Because such a closure must move axially before the bead engages and lifts the disk, the sidewall heights of the closure and container finish have heretofore been significantly greater than those of a corresponding vacuum sealing closure having an integral cover rather than a floating disk; the "float" has increased the sidewall height of the cap skirt and the container finish.
The provision of tamper evidencing means on a floating disk closure usually requires still more vertical height. Usually such means respond to twisting of the closure by rupture or break-off of a band or other easily visible feature; if such a rupture or break is visible while the product is on the shelf, tampering is apparent. As it is turned, a closure having a tamper evidencing means should first break the tamper evidencing means, and then after further turning, lift the disk. (It is desirable to have the tamper evidencing means break before the disk is lifted in order to insure that the possible tampering will be made evident even before the vacuum could have been broken.) Thus the provision of tamper evidencing means further increases the overall height of floating disk closures. A closure having both tamper evidencing means and a floating disk has heretofore had a relatively large "aspect ratio" or vertical height for a given diameter, in comparison to a corresponding closure without those features. In consequence of the greater height, relatively more material has been required for such a closure and container.